Docstoc

ITRC Subcritical Contraction Design Spreadsheet - Irrigation

Document Sample
ITRC Subcritical Contraction Design Spreadsheet - Irrigation Powered By Docstoc
					Designing a Subcritical Contraction for Upward-Looking (and side-looking) ADVMs
            Created by D.J. Howes, Ph.D., P.E., Irrigation Training and Research Center, Cal Poly San Luis Obispo.
            Version 1.0, created 7/28/2011


This spreadsheet was created to assist with the hydraulic design of the subcritical contraction for improving open
channel flow measurement with a vertical profiling upward-looking ADVM such as a SonTek-SW (shallow water).
Details on the design of this contraction can be found at:

            Howes, D. J., C. M. Burt and B. F. Sanders (2010). "Subcritical contraction for improved open channel
            flow measurement accuracy with an upward-looking ADVM." Journal of Irrigation and Drainage
            Engineering, 136 (9): 617-626.


Equipment and Experience

 Very precise measurements are NOT required for the hydraulic design of the subcritical contraction.
 Measurements to within 0.1 ft should be sufficient. Elevation measurements can simply be relative to each
 other, they do not have to be tied to a benchmark.

 Ideally the channel will be dewatered when the survey measurements are taken. If this is not possible, all safety
 precautions should be in place for open water surveying. This includes the use of life vests, tethers to secure
 personnel, boats to move across the channel, etc. If you do not feel comfortable with this type of surveying a
 professional should be consulted.

 Survey Equipment
          Rod and level (laser level is okay)
          Tape

 Design
            PC Computer
            Software:
             Microsoft EXCEL
             AutoCAD (optional but recommended)

Procedure
                                                               1
                                                                                                                     2
 I. Surveying
           A. Elevations                                             3
             1. Top of Bank                                                                     B
             2. High water mark                                     1
             3. Estimated typical or average water level
                                                                                      hmax
                                                                             Channel Cross Section
                                                                         m   4
             4. Estimated minimum water depth                                              /2
             5. Channel bottom                                                         5
                                                                                                w
            Notes: If the channel bottom is not level, 3-5 elevations should be collected and averaged
            for the channel bottom elevation.



cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                                1/9
           B. Distances/Lengths
             1. Length across the top of the channel                              1
             2. Bottom width of channel


                                                                                  B
                                                              1
                                                                              hma
                                                                  m
                                                                              x/2
                                                                                w
                                                                                  2


           C. Maximum Flow Rate
             1. Obtain an estimate of maximum flow rate from the canal operator and water master
             2. If future flows could be higher, overestimate the maximum flow rate.


                         - The subcritical contraction works best under low Froude Numbers. Therefore it is always
                         better to overdesign the contraction (i.e. overestimate the maximum flow rate)


           D. Data Entry
            1. Enter elevation and length data into the Survey_English worksheet (if you are using
            English/Imperial Units)
            2. All units in the Survey_English worksheet are in feet.

             3. Enter the maximum expected flow rate into the Design_English worksheet in cubic feet per second

II. Contraction Design
           Once the survey measurements have been collected and entered into the Survey_English worksheet
           the hydraulic design of the contraction can be conducted.

           A) Contraction Inlet and Throat Length
            There are only 4 values that will be entered to obtain these design parameters
             1) The maximum flow rate in cell C6
             2) The target water depth in cell C7. This should be the expected water depth at the maximum flow
             rate. In many cases this value may be assumed to be equal to the maximum water depth. However, if
             the water depth is expected to be lower than the maximum water depth at the maximum flow rate,
             the expected value should be entered.
            3) ADVM beam angle which is typically 45 deg but can be 25 degrees.
            4) Contraction ratio will be found through manual iterations.
                     Criteria for the contraction ratio includes:
                        a) Maximum Froude Number (Fmax) must be less than 0.5
                        b) Contraction Ratio must be less than or equal to 0.75
           Hints:       - In rectangular channels, a contraction ratio of 0.5 is a good starting point.
                        - In trapezoidal channels, a contraction ratio where the contraction opening (b) is equal
                        to the bottom width of the original channel (w) is a good starting point.


cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                            2/9
          As shown in the figure below, if the criteria are met OKAY will appear. If it is not possible to meet the
          design criteria, alternative flow measurement devices should be investigated.




           4) Issues that can arise as shown in the images below

                      - If a contraction ratio must be selected where the contraction opening (b) is GREATER
                      THAN the bottom width of the original channel (w), but within 4 feet (~1.2 meter) care
                      must be taken to construct an appropriate transition (See III.B below)
                      - If b is more than 4 feet greater than w, major channel modifications are required in
                      order to install the subcritical contraction (see III.B below).




           5) The contraction inlet and throat length design parameters are computed in the spreadsheet and
           can be found under the Contraction Design heading
cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                             3/9
          B) Contraction Exit
                     The contraction exit should not impact the flow measurement accuracy therefore it is
                     designers choice. The spreadsheet provides design specification for two possible options, a
                     straight flared exit and a rounded exit transition. Designers may choose one of these or a
                     different option.




                      The last - Contraction is to compute
          C) Velocity Equationdesign portionCoefficient. the contraction coefficient. This should be completed
                      after the ADVM has been mounted on the channel bottom. Details on the mount height
                      and the height of the sensor are required to compute the coefficient.

                     1) Enter the ADVM mount height from the channel bottom.
                     2) Enter the ADVM sensor height

                     3) Enter the blanking distance selected in the ADVM software (typically about 7 cm)

                     The contraction coefficient, Csc, will be computed. This should be incorporated into the
                     ADVM software to accurately compute the cross sectional average velocity.


cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                       4/9
III. Other Design Items
            A) Mounting the ADVM

                     The ADVM should be mounted so that it can be removed and cleaned easily while water is
                     flowing in the channel. The bracket that allows the ADVM to be removed should be outside
                     of the flow path so the velocity profiles are not interrupted. The following images show
                     approximately 1/2 inch plate used to mount the ADVM in the center of the channel. The
                     plate lies flat on the channel bottom and the contraction is constructed with a recess along
                     the channel wall so the bracket is not in the flow path.




          B) Contraction Width versus Bottom Width

                     If the contraction opening is greater than the bottom width, issues can arise in the
                     effectiveness of the contraction. If the contraction opening (b) greater than the bottom
                     width (w) but they are within 3 feet, a smooth transition must be created. If the difference
                     is greater than 3 feet an alternative channel geometry should be constructed.
                     Figure below shows a transition to wider bottom (2 ft difference).




cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                           5/9
           The figure below shows a modified section. New section is rectangular for approximately 4 contraction
           widths upstream, transitions back to a trapezoidal channel downstream of the contraction.




                                                     Existing
                                                     Entrance




                                                                            Sharp
                                                                          Contraction




FINAL NOTE: It is recommended that the contraction design be drawn in AutoCAD to scale to obtain perspective
on how the design will fit into the channel.


cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                      6/9
Original Channel Dimensions
Location:                 Enter Name of Flow Measurement Site
 i) Vertical Dimensions
                                                         Survey Elevation
            Location                                        (relative)              1
            1. Channel Top of Bank                                     38.80   ft                                       2
            2. High Water Mark                                         38.00   ft
                                                                                        3
            3. Estimated Average Water Level                           37.50   ft                                B
            4. Estimated Minimum Water Level                           36.50   ft       1
            5. Channel Bottom                                          34.60   ft                        hmax
                                                                                                Channel Cross Section
                                                                                            m   4
                                                                                                            /2
                         Computed Depths
            Total Channel Depth                                         4.20 ft                         5
            Depth at High Water Mark (hmax)                             3.40 ft                                  w
            Estimated Average Water Depth                               2.90 ft
                                                                                                                 1
ii) Horizontal Dimensions
            Channel Widths                                   Measurement
            1. Width at Top of Bank                                   20.00 ft
            2. Bottom Width                                           11.00 ft                                   B
                                                                                        1
                         Computed Values                                                                 hmax
                                                                                                Channel Cross Section
            (Top W-Bottom W)/2                                           4.5 ft             m
                                                                                                         /2
            m                                                           1.07                                  w
            Channel width at 0.5*Depth at hmax, (B)                    14.64 ft
                                                                                                                 2




                                      B
                 1                                    hmax

                     m           hmax/2


                                      w




cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx                                                                                   7
Subcritical Contraction Design for Improving Upward-Looking ADVM Accuracy
Version 1.2        English Units     Only change values in cells that are NOT gray.
    Location:                      Enter Name of Flow Measurement Site
Dimensions (Grey cells are brought in from the Survey worksheet)
Description                                English Units
Maximum Flow Rate                            180 cfs                                                                                             B
Target Water Depth                          3.40 ft         Normally equal to maximum water depth                      1                                    hmax
ADVM θbeam (beam angle)                       45 degrees    (typically 45 degrees but could be 25 deg.)
                                                                                                                            m              hmax/2
Bottom width (w)                            11.00 ft
Bank Slope (m)                               1.07
Minimum Depth (hmin)                         1.90 ft             OKAY         Should be greater than ~1.3 ft                                     w
Max Depth (hmax)                             3.40   ft
Top of Bank (TOB) Depth                      4.20   ft                                                                                         B
Width at hmax                               18.29   ft
                                                                                                                       L1
TOB width (computed)                        20.00   ft           OKAY         check
B                                           14.64   ft      (if error check measurements)


Adjust Contraction Ratio until the following criteria are met                                       L2           1.5
  Fmax < 0.5 and Contraction Ratio < 0.75                                                                          1.0

                                                                                                                                     s                             Lt = 1.25*(B-b)
Contraction Ratio (Cr=b/B)                     0.75              OKAY         check
                                                                                                         33.7˚          a

Contraction Design
Description                                English Units
  B                                        14.64 ft
  b                                        10.98 ft                    OKAY                 LADVM = hmax*tan(θbeam)
  Rentrance                                 6.04 ft
  θ                                         33.7 degrees                                                                                             ADVM
  θ (radians)                               0.59 radians                                                                                                    LTL = 2*hmax*tan(θbeam)
  s                                         3.55 ft
  Lt                                        4.58 ft
  a                                         3.35 ft                                                                                            b
  L1                                        0.82 ft
  L2                                        1.22 ft
  LADVM                                     3.40 ft         based on beam angle
  MinimumThroat Length (Ltl)                 6.80 ft        based on beam angle
Max. Velocity in Contraction                 4.82 ft/s                                                                                                        Flow
Max. Froude Number (Fmax)                   0.461                                                         Flow


Contraction Exit Options
Description                                English Units
Straight Exit
                                                                                                                                1                                                     Rexit
                   Lexit                    13.53 ft
                   Wexit                     4.51 ft                       Lexit
                                                                                                                                     6
Rounded Exit
                   Rexit                     4.51 ft

Velocity Relationship Equation

 V  C scVADVM                                                                              Wexit
                                       2
                      zb                     zb                                                                                        b
        C sc  0.0121
                     z               - 0.026
                                             z             + 0.9625
                                                            
                      b max                  b max       
Description                         English Units
Mounting height                            0.07 ft
Upward-looking ADVM height                 0.20 ft
Blanking distance                          0.23 ft                                                                                       hmax
Buffer Distance, zb                        0.49 ft

zbmax                                        0.66 ft
Csc                                        0.9498
                                                                                                                                    Front View




English Units                                                               cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx
Subcritical Contraction Design for Improving Side-Looking ADVM Accuracy
Version 1.2          English Units     Only change values in cells that are NOT gray.
    Location:                        Enter Name of Flow Measurement Site
Dimensions (Grey cells are brought in from the Survey worksheet)
Description                                 English Units
Maximum Flow Rate                             180 cfs                                                                                              B
Target Water Depth                           3.40 ft          Normally equal to maximum water depth                      1                                    hmax
ADVM θbeam (beam angle)                        25 degrees     (typically 45 degrees but could be 25 deg.)
                                                                                                                              m              hmax/2
Bottom width (w)                            11.00 ft
Bank Slope (m)                               1.07
Minimum Depth (hmin)                         1.90 ft               OKAY         Should be greater than ~1.3 ft                                     w
Max Depth (hmax)                             3.40   ft
Top of Bank (TOB) Depth                      4.20   ft                                                                                           B
Width at hmax                               18.29   ft
TOB width (computed)                        20.00   ft             OKAY         check                                    L1
B                                           14.64   ft        (if error check measurements)


Adjust Contraction Ratio until the following criteria are met                                         L2           1.5
  Fmax < 0.5 and Contraction Ratio < 0.75                                                                            1.0

                                                                                                                                                                     Lt = 1.25*(B-b)
Contraction Ratio (Cr=b/B)                     0.75                OKAY         check                                                   s
                                                                                                           33.7˚          a

Contraction Design
Description                                 English Units
  B                                         14.64 ft
  b                                         10.98 ft                     OKAY                 LADVM = b*tan(θbeam)+1
  Rentrance                                  6.04 ft
  θ                                          33.7 degrees                                                                                              ADVM
  θ (radians)                                0.59 radians                                                                                                     LTL = 2*b*tan(θbeam)+3
  s                                          3.55 ft
  Lt                                         4.58 ft
  a                                          3.35 ft                                                                                             b
  L1                                         0.82 ft
  L2                                         1.22 ft
  LADVM                                      6.12 ft          based on beam angle
  MinimumThroat Length (Ltl)                13.24 ft          based on beam angle
Max. Velocity in Contraction                 4.82 ft/s                                                                                                          Flow
Max. Froude Number (Fmax)                   0.461                                                           Flow


Contraction Exit Options
Description                                 English Units
Straight Exit
                                                                                                                                  1                                                    Rexit
                     Lexit                  13.53 ft
                     Wexit                   4.51 ft                         Lexit
                                                                                                                                       6
Rounded Exit
                     Rexit                   4.51 ft

Velocity Relationship Equation

 V  C scVADVM                                                                                Wexit


                              0 . 955
                                                                                                                                             b
      C         sc
With buffer distance of 10%*b from both sides of contraction or 1 foot, whichever is less


                                                                                                                                            hmax




                                                                                                                                      Front View




English Units                                                                 cd01b346-9e45-4e6b-8f78-9537bb85d2e5.xlsx

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:2
posted:4/21/2013
language:English
pages:9