ME 120 Experimental Methods Laboratory

							       ME 120 Experimental Methods
       Laboratory



              Ananda Mysore




Spring 2008             Ananda Mysore
Sound Pressure Measurements

    Objectives
             Become familiar with hardware and techniques to
              measure sound pressure
             Measure Sound level of various size fan modules
             Calculate S/N of acoustic measurements
             Fan Noise = f(size & operating speed)




                           ME 120 Experimental Methods Laboratory
Spring 2008                           Ananda Mysore
Basics

    @ 20deg C Vel of sound = 344m/sec
    Audible frequencies - 20 to 20000Hz
    Bruel &Kjaer 2236 Precision Integrating sound level
     meter
             Condenser Microphone
                    Thin metal foil stretched like drumhead over a frame. Pressure
                     changes > diaphragm movement > changes in capacitance>
                     milli-volt signal > pre amplifier > other amplifier stages > A/D
                     converter
                 Free-field response Microphone > one direction only
                 Random-incidence Microphone > Equally from all directions



                                ME 120 Experimental Methods Laboratory
Spring 2008                                Ananda Mysore
Basics

    SPL is a measurement of sound strength on a logarithmic
     scale
    L = SPL = 10 log(Signal Power/Ref Power)
    L = 10 log (Prms/Pref)^2
    L = 20 log(Prms/Pref)
    Pref = Ref value for sound pressure in air = 20 micro pascals
    Sound level meter computes Prms in db based on sound
     pressure over a period of time




                        ME 120 Experimental Methods Laboratory
Spring 2008                        Ananda Mysore
Vibration Measurements

    Objectives
                 Find first natural frequency of a cantilever beam using both
                  theoretical and experimental techniques and to estimate
                  first mode’s damping
                 Find the second natural frequency of a cantilever beam by
                  locating 180deg phase shift




                              ME 120 Experimental Methods Laboratory
Spring 2008                              Ananda Mysore
Basics
                                          Every physical body has infinite
                                          number of natural frequencies




                                               ωn1< ωn2< ωn3




              ME 120 Experimental Methods Laboratory
Spring 2008              Ananda Mysore
Measurement

    Frequency measurement using:
         Electromechanical shake table
         Accelerometers
         Oscilloscope
    Goals
             Ratio of the amplitude of the end of the beam
             Phase relationship between Input and output signals
             Plot Phase angle and amplitude ratio vs frequency
             Estimate first mode’s damping ratio
             Use phase change to locate the second modal frequency
             At constant frequency measure phase at different amplitude ratios
             Repeat the experiment with plastic beam


                              ME 120 Experimental Methods Laboratory
Spring 2008                              Ananda Mysore
Static Deflection Measurement

    Objective
                 Compare the results from static beam deflection
                  measurement with the result computed from the analytical
                  prediction




                              ME 120 Experimental Methods Laboratory
Spring 2008                              Ananda Mysore
Basics

    Area moment of inertia > resistance to bending
    For a rectangular cross section area moment of inertia:
                 I = bh^3/12 b= width and h = height
    LVDT: Linear Variable Differential Transformer




Source: http://www.macrosensors.com/


                                       ME 120 Experimental Methods Laboratory
Spring 2008                                       Ananda Mysore
Viscosity Measurement

    Objectives
             Learn to operate and take data with a rotating spindle
              type viscometer
             Investigate 10W-30 oil is a Newtonian or non-Newtonian
              fluid
             Determine the viscosity of 10W-30 motor oil as a
              function of temperature




                           ME 120 Experimental Methods Laboratory
Spring 2008                           Ananda Mysore
Basics

    A Newtonian fluid :
              is a fluid that flows like water—its shear stress versus rate of strain curve is
               linear and passes through the origin.
              Viscosity depends on temperature and pressure and not on the forces acting
               on it
                                                                    Application of non-Newtonian fluids:
                                                                    Body armor development(bullet proof
                                                                    vest) - shear resistant vs paint -
                                                                    shear thinning




          Source:http://en.wikipedia.org/wiki/Image:Non-Newtonian_fluid.PNG




                                             ME 120 Experimental Methods Laboratory
Spring 2008                                             Ananda Mysore