Venturi Effect

Document Sample
Venturi Effect Powered By Docstoc
					                              Venturi: Potential Flow

                                 G.Rozza and A.T.Patera
                   in collaboration with D.B.P Huynh and NC Nguyen

                                       March 28, 2008

1    Engineering Motivation

Flows in ducts, channels, and pipelines are of great interest in fluid mechanics applications
especially when flows can be studied in a parametrized geometrical configuration. This Worked
Problem considers a 2D potential flow into a Venturi channel and is an example of a design of
a parametrized fluidic device as an element of a more complex modular fluidic system (if we
adopt a more complex fluid model).
   In this Worked Problem we illustrate the calculation of pressure and velocity by the Bernoulli
Theorem and the curvy geometry parametrization. We also illustrate some features dealing
with potential flows (i ) an error bound on velocity and pressure (and not only on the potential
solution) and (ii ) visualization of the flow field (streamline) and pressure contours.

2    Physical Principles

The physical principles illustrated in this Worked Problem deal with ideal fluid mechanics
(non-viscous fluids) modelled by potential flows and based on the Bernoulli Theorem. Velocity
and pressure are influenced by the channel/constriction configuration (i.e. height and length
of the throat and radius of curvature of the connection). Gravitational effects or other force
fields could be applied.

    The Venturi tube is an example of the application of Bernoulli’s principle, in the case of
incompressible flow through a tube or pipe with a constriction in it. The fluid velocity must
increase through the constriction to satisfy the equation of continuity, while its pressure must
decrease due to conservation of energy. The gain in kinetic energy is supplied by a drop in
pressure or a pressure gradient force. The effect is named after Giovanni Battista Venturi,
(1746–1822), an Italian physicist.

   The limiting case of the Venturi effect is choked flow, in which a constriction in a pipe or
channel limits the total flow rate through the channel because the pressure cannot drop below

                                                                                          Γ3         Γ9
                                                                                     Γ4                   Γ8
                  (4 − µ3 , 1) (4 + µ2 + µ3 , 1)                                    Γ5                     Γ7
    (0, 1)                                           (8 + µ2 , 1)             Γ2                                Γ10
                                                                    Γ1                         R1                   Γ11
    (0, 0)                                           (8 + µ2 , 0)             Γ14              Γ13              Γ12
                       (4, 0)   (4 + µ2 , 0)
       (4, 1 − µ3 )
       (4,                                  (4 + µ2 , 1 − µ3 )                       Γ6
        (4, µ1 + µ3 )
        (4,                               (4 + µ2 , µ1 + µ3 )
          (4 + µ3 , µ1 )                (4 + µ2 − µ3 , µ1 )

             Figure 1: Parametrized Geometry                              Figure 2: Domain Boundaries

zero in the constriction. Choked flow is used to control the delivery rate of water and other
fluids through valves.

    Examples of the Venturi effect are everywhere: in the capillaries of the human circulatory
system; in large cities where wind is forced between buildings; in inspirators that mix air and
flammable gas in barbecues, gas stoves, Bunsen burners and airbrushes; in water aspirators
that produce a partial vacuum using the kinetic energy from the faucet water pressure; in
steam siphon using the kinetic energy from the steam pressure to create a partial vacuum;
in atomizers that disperse perfume or spray paint (i.e., from a spray gun); in firefighting
nozzles and extinguishers using foam; in compressed air operated industrial vacuum cleaners;
in injectors (or sometimes called ejectors). Some carburetors are built use the Venturi effect to
suck gasoline into an engine’s air intake, and some vaporizers are designed to use the Venturi
effect to optimize efficiency.

3     Problem Description

We consider the physical domain Ω(µ) shown in Figure 1. Here x = (x1 , x2 ) denotes a point
in Ω(µ), non-dimensionalized with respect to height of the inlet L. Note that a tilde˜denotes
dimensional quantities, and the absence of a tilde signals a non-dimensional quantity. We
identify in Figure 2 the region R1 , representing the Venturi channel (inlet, connection, throat,
connection, oulet).

    In this problem the boundary segments Γ3 , Γ5 , Γ7 , Γ9 are curved (all other boundary seg-
ments and internal interfaces are straight lines). The segment Γ3 is given by the parametriza-
                    x1        4 − µ3         1 0      µ3 0        cos (πt)
                         =              +                                     ,
                    x2        1 − µ3         0 1      0 µ3        sin (πt)
                                        O1 (µ)         Q1 (µ)       S 1 (µ)

where t ∈ [0, 1/2]. The segment Γ5 is given by the parametrization

                         x1           4 + µ3            1 0         µ3 0            cos (πt)
                                =                  +                                                 ,
                         x2           µ1 + µ3           0 1         0 µ3            sin (πt)
                                        O2 (µ)          Q2 (µ)      S 2 (µ)

where t ∈ [1, 3/2]. The segment Γ7 is given by the parametrization
                   x1           4 + µ2 − µ3           1 0      µ3 0        cos (πt)
                        =                         +                                   ,
                   x2             µ1 + µ3             0 1      0 µ3        sin (πt)
                                   O3 (µ)             Q3 (µ)    S 3 (µ)

where t ∈ [3/2, 2]. The segment Γ9 is given by the parametrization
                  x1            4 + µ2 + µ3           1 0       µ3 0       cos (πt)
                        =                         +                                       ,
                  x2               1 − µ3             0 1       0 µ3       sin (πt)
                                   O4 (µ)             Q4 (µ)     S 4 (µ)

where t ∈ [1/2, 1]. The Venturi element could be integrated into a more complex modular

    We consider here P = 3 parameters. Here µ1 , µ2 , µ3 are geometry parameters defined in
Figure 1; µ1 is the height of the throat (i.e., the central narrow part of the Venturi channel),
µ2 is the length of the narrow part of the channel, and µ3 is the radius used to smooth the
connections between the inlet (and the outlet) with the central throat. The parameter domain
is given by D = [0.25, 0.5] × [2, 4] × [0.1, 0.2].

   The non-dimensional potential is denoted ϕ(µ) and is defined as ϕ(µ) =                   ee ,   where φ
is the dimensional potential, LU is the reference potential and U is a dimensional reference
velocity. We will use the inlet velocity vin as reference in the following.

   The governing equation for ϕ(µ) is a generalized Laplacian:
                                 ∂      1 0        ∂
                            −                         ϕ(µ)     =0    in R1 ,
                                ∂xi     0 1       ∂xj

with summation (i, j = 1, 2) over repeated indices.

    Velocity is given by the gradient of the potential function (being a conservative field):
˜     ˜                     ˜                                                        ˜
v = vin ϕ(µ), where vin is the inlet dimensional velocity. By fixing the pressure pin into
a point (i.e. for example pin = 1 at the inlet) and having computed the velocity field in
                   ˜      ˜
all the domain v = vin ϕ(µ) (computing the gradient of the potential function) we may
                                                     ˜ 1˜ v                        1
get the pressure p by applying Bernoulli Theorem: p + 2 ρ(˜in )2 | ϕ(µ)|2 = pin + 2 ρ(˜in )2 ,
                     ˜                                                       ˜         v
where ρ is the density (also the Bernoulli equation can be non-dimensionalized). The non-
dimensional velocity is given by v = ϕ(µ) and the non-dimensional pressure is given by
p = (˜ − pin )/( 1 ρvin ) = (1 − | ϕ(µ)|2 ).
      p ˜        2 ˜˜

    We must also impose interface and boundary conditions. On all internal interfaces (in-
terior boundaries of regions), we impose continuity of the potential ϕ(µ) and its gradient
ni κij     ϕ(µ), where ni and ei denote normal and tangential unit vectors.

   We show in Figure 2 the boundaries of the domain. On boundary Γ11 we impose homoge-
neous Dirichlet conditions ϕ(µ) = 0, while on boundaries Γ2 , Γ3 , Γ4 , Γ5 , Γ6 , Γ7 , Γ8 , Γ9 , Γ10 ,

                                   Figure 3: Finite Element Mesh

Γ12 , Γ13 and Γ14 we impose homogeneous Neumann conditions, ni κij   ϕ(µ) = 0 (i.e., zero
velocity). In addition we impose non-homogeneous Neumann condition,
                                    ni κ1
                                        ij       (µ) = 1 on Γ1 ,

(i.e., imposition of the velocity at the inlet).

    For this problem the output of interest is provided by the visualization of velocity field (by
streamlines) and/or pressure contour field. The error bound is computed on the pressure and
on the velocity.

   This problem is then modeled by the P1 finite element (FE) discretization over the trian-
gulation shown in Figure 3; the FE space contains Nt = 3137 degrees of freedom. This FE
approximation is typically too slow for many applications, and we hence approximate the FE
prediction for the output and field variable by the reduced basis (RB) method.

   The user can obtain the RB prediction for the output and field variable (visualization) —
as well as a rigorous error bound for the difference between the RB and FE predictions —
through our webserver. (Users who wish to run on their own computers and who have already
downloaded our rbMIT software package can also create the RB approximation on their own
computer from the rbU file.)

4     Additional Information for Visualization and Velocity/Pres-
      sure Pointwise Calculation

4.1   Vis_RB_Bernoulli and Online_RB_pointwise_Bernoulli

The special visualization function provided with this problem Vis_RB_Bernoulli (i ) plots not
only the potential solution but also the velocity field by streamlines and the pressure contours
and (ii ) computes an error estimator on the velocity and on the pressure and not only on the
potential solution. The general call for the function is


where probname='venturi' and mu is the vector parameter [mu1,mu2,mu3].

    In addition,

[u, v, p, DeltaNV, DeltaNP]=Online_RB_pointwise_Bernoulli(probname, mu, xo)

is the function to call to get pointwise quantities into an internal point (xo=[x,y]) or more
points (xo=[x1,y1;x2,y2;x3,y3,...]) of the domain. Just specify the value (mu) of the
parameter and 'probname'.

    As regards outputs: u is the (pointwise) horizontal velocity; v is the (pointwise) vertical
velocity; and p is the (pointwise) pressure at the selected point(s). DeltaNV and DeltaNP are
the L1 associated pointwise error bounds on velocity and pressure, respectively.

    The functions Vis_RB_Bernoulli and Online_RB_pointwise_Bernoulli are provided in
the rbMIT_data folder along with the other data and functions related to this Worked Problem.

4.2   Some Provided Pre-Computed Solutions

We provide in

[u, v, p, DeltaNV, DeltaNP, varargout{1}]=Online_RB_pointwise_Bernoulli(...
... probname, mu, 'preset',varargin{1},varargin{2})

a set of “pre-computed” solutions on crucial points in the domain to extract velocity and/or
pressure profiles. Two options are available: (i ) varargin{1}=0 and varargin{2}=x2;
(ii ) varargin{1}=x1 and varargin{2}=0.

    With varargin{1}=0 and varargin{2}=x2 some pre-computed solutions are provided at
an imaginary line at a constant height in the throat. The value of the height can be, for exam-
ple, x2 = 0.05, 0.1, 0.15, 0.20, ..., 0.45. With varargin{1}=x1 and varargin{2}=0
some pre-computed solutions are provided on an imaginary line representing a section of the
throat. The value of x2, for example, can be 4.0, 4.25, 4.5, . . . , 7.75, 8.0, and it should be
contained inside the domain consistently with the value chosen for mu2; varargout{1}=xo.

5     Pedagogy Questions
 Q1. Predict/confirm the shape of the velocity profile and the pressure field using a 1D model
     based on continuity equation and Bernoulli’s theorem.
       Hint: You can consider 3 different sections — the inlet, the middle of the
     throat, and the outlet — and then compute the pressure drop and the pressure
     recovery. To answer this question you will need to use the additional function
     Online_RB_pointwise_Bernoulli provided to compute pointwise velocity/pressure and
     the associated L1 error bound.)
 Q2. What is the pressure behavior and the velocity profile at high curvature points in the
     throat-inlet connection and in the throat-outlet connection?
       Hint: To answer this question you will need to use the additional function

    Online_RB_pointwise_Bernoulli provided to compute pointwise velocity/pressure and
    the associated L1 error bound.)

Q3. How does the height of the throat (i.e., mu3 parameter) affect the low pressure at the
    “apex” of the curve in the transition region?

Q4. How long does it take the flow to become 1D in the throat region? What is the influence
    (if any) of the length of the throat (i.e., mu2 parameter)?