FLUENT在气动噪声问题上的处理方法

【实例简介】
气动声学在很多工业领域中倍受关注，模拟起来却相当困难，如今，使用FLUENT 可以有多种方法计算由非稳态压力脉动引起的噪音，瞬态大涡模拟（LES）预测的表面压力可以使用FLUENT内嵌的快速傅立叶变换（FFT）工具转换成频谱。
Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA 2m 2d, dp, pbns, rk Figure 2: Contours of Static Pressure(Steady State 3g2e+01 2d, dp, pbns, rk Figure 3: Contours of Velocity Magnitude( Steady State Fluent Inc. March 12. 2008 3 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA) ep 2: Models 1. Select unsteady solver Define odes→ Solver (a)Select Unsteady in the Time list (b) Select 2nd-order-implicit in the Unsteady formulation list (c) Retain the default settings for other parameters (d)Click Ok to close the Solver panel 2. Define the viscous model Define odels IsCoUs (a)Select Non-Equilibrium Wall Functions in the Near-Wall Treatment list (b) Retain the default settigns for other parameters (c) Click OK to close the Viscous Model panel Near-Wall Treatment predicts good separation and re-attachment points Step 3: Materials Defin Materials 1. Select ideal-gas from the density drop-down list 2. Retain the default values for other parameters 3. Click Change/Create and close the Materials panel Ideal gas law is good in predicting the small changes in the pressure Step 4: Solution 1. Monitor the static pressure on point-1 and point-2 Monitors—→ Surface a)Enter 2 for the Surface Monitors (b) Enablc Plot and Print options for monitor-1 and monitor-2 (c) Select Time Step from the When list (d) Click Define. for monitor-1 to open Define Surface Monitor panel Fluent Inc. March 12, 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA a Define surface monite Name Report of monitor-t Pressure Report Type Static pressure vertexAwerage Surfaces X Axis outlet Flow time oint-I 2 Plot windo Fille name monitor-1out Curves Axes Cancel Help i. Select Vertex Average from the Report Type drop-down list i1. Select Flow Time from the X Axis drop-down list Enter 1 for Plot window iv. Sclcct point-1 from the Surfaces sclcction list (e) similarly, specify the surface monitor parameters for point-2 2. Start the calculations using the following settings ove (a) Enter 3e-04 s for Time Step Size The ecpected time step size for this problem is of the size of about 1/ 10th of the time period. The time period depends on the frequency (f)which is calcalated using the following equation TVVIL+2 eed of sound S= Area of the orifice of the resonator V= Volame of the resonator L=Length of the connection between the resonator and the free flow area Dh Hydraulic diameter of the orifice For this geometry, the estimated frequency is about120 H2 b) Enter 250 for the Number of Time Steps (c Enter 50 for Max Iterations per Time Step (d)Click Apply Fluent Inc. March 12. 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA) (e) Read the scheme file(stptmstp4. scm) e Read→ Scheme This file activates a alternative convergence criteria. For acoustic simulations with Caa it is obligatory that the pressure is completely converged at the reciever position. FLUEnT compares the monitor quantities within the last n-defined it- eraliors lo judge i che deviation is snaller than a y-delired deviation (f)Specify the number of previous iterations from which monitor values of each quantity used are saved and compared to the current(latest)value(include the parenthesis) (set! stptmstp-n 5) (g) Specify the relative(the smaller of two valucs in any comparison) diffcrcncc by which any of the older monitor values(for a selected monitor qauntity)may differ from the newest value (set! stptmstp-maxrelchng 1 e-02) (h) Define the execute commands SolveExecute Commands Enter(stptmstp-resetvalues)for the first command and select Time Step from the drop-down list ii. Enter(stptmstp-chckcnvrg "/report/surface-integrals vertex-avg point-1 O pressure")and select Iteration from the drop-down list Click oK (i)Click Iterate to start the calculations The iterations will take a long time to complete. You can skip this simulation af- ter few time steps and read the files(transient cas. gz and transient dat. gz provided with this tutorial. These files contain the data for the fow time of 0.22 seconds. As seen in Figures f and 5, no pressure fuctuations are present at this stage. The oscillations of the static pressure at both monitor points has reached a constant valuc The RANS-simalation is a good starting point for Large Eddy Simulation. If you choose to use the steady solution as initial condition for les, use the Tul command solve initializerinit-instantaneous-vel provides to get a more realistic instantaneous velocity field. The usage of LES for acoustic simulations is obliga tory. The nect two pictures compare the static pressure obtained with RANS and Large Eddy Simulation for a complete simulation until 0.525 seconds. Obviously the he-epsilon model underpredicts the strong pressure oscillation after reaching a dymamically steady state(>0.3 s) due to its dissipative character. Under- predicted pressure oscillations lead to underpredicted sound pressure level which means the acoustic noise is more gentle Fluent Inc. March 12, 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA 1E Average values 800 F|MTim能e nce hlstory of static Pressure cn polnt-1 (TIme=7.5000e-02) 2d, d pbns, rke, unsteady Figure 4: Convergence History of Static Pressure on Point-1(Transient Average 0.00 trace erte 10.0 alues ep Conergence history of Static pressure on point-2 Time=T.5000E-027 Figure 5: Convergence History of Static Pressure on Point-2(Transient Fluent Inc. March 12. 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA) Step 5: Enable Large Eddy Simulation 1. Enter the following TUI command in the FLUEnT console ( rpsetvar’1es-2d?#) 2. Enable large eddy simulation effects The k-epsilon model cannot resolve very small pressure fuctuations for aeroacoustic due to its dissipative character. Use Large eddy simulation to overcome this roblem Define ModelsViscous ous Model Model Model constants id Cuale .325 palart-allmaras [1 eqn C k-epsilon 2 egn) Energy Prandtl Number C k-omega 2 egnl .85 Reynolds stress eqn C Detached Eddy Simulation fall Prandtl Numbel c Large Eddy Simulation (LES) .85 Subgrid-Scale Model Smagorinsky-Lilly WALE C Kinetic-Energy Trans port User-Defined functions Subgrid-scale Turbulent viscosity Options nonte 厂ⅶ sous Heating Cancel Help (a) Enable large eddy Simulation(LES) in the Model list (b) Enable WALE in the Subgrid-Scale Model list. (c) Click OK to closc the Viscous Model panc An, Information panel will appear, warning about bounded central-deferemcing be ing default for momentum with LES/DES Information Warning Bounded Central Differencing is default for momentum with LESPDES Fluent Inc. March 12, 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA (d)Click OK to close the Information pa 3. Retain default discretization schemes and under-relaxation factors Se olve H ControlsSolution 4. Enable writing of two surface monitors and specify file names as monitor-les-1 out and monitor-les-2 out for monitor plots of point-I and point-2 respective tor Solve Monitors→ Surface To account for stochastic components of the flow, FLUent provides two algorithms These algorithms model the fluctuating velocity at velocity inlets. With the spec tral synthesizer the fluctuating velocity components are computed by synthesizing a divergence-free velocity-vector field from the summation of Fourier harmonics Enable the spectral synthesizer Define Boundary Conditions 2 Velocity Inlet Zone name inle Momentum Thermal Radiation Species DPM Multiphase UDS I Velocity Specification Method Magnitude, Normal to Boundary Reference Frame Absolute Velocity Magnitude (m/=]27.78 constant Fluctuating Velocity Algorithm Spectral Synthesizer lurbulence Specification Method Intensity and Hydraulic Diameter Turbulent Intensity (9%]1 Hydraulic diameter [m Reynalds-Stress Specification Method K or Turbulent Intensity OK Cancel Help (a) select inlet in the Zone list and click S i. Select Spectral Synthesizer from the Fluctuating Velocity algorithm drop-down ii. Retain the default values for other parameters iii. Click ok to close the velocity Inlet panel (b) close the boundary Conditions panel Fluent Inc. March 12. 2008 Modeling Aeroacoustics for a Helmholtz Resonator Using the Direct Method (CAA) Typically il lakies a long line lo gel a dynamically sleady sale. Addilionally, the simulated(and recorded for FFT)fow time depends on the minimum frequcncy in the following relationship 10 flowtime= minimum frequency The standard transient scheme (iterative time advancement) requires a considerable amounl o/ conpulaional eorl due lo u large nunberofouler ilerulioTs per orned Jor each time-step. To accelerate the simulation, the NITA (non-iterative time advance ment) scheme is am alternative 6. Set the solver parameters Define Models→ Solver (a) Enable Non-lterative Time Advancement in the Transient Controls lisl (b) Click Ok to close the Solver panel 7. Set the solution parameters Solve Controls→ Solution (a)Select Fractional Step from the Pressure-Velocity Coupling drop-down list (b)Click OK to close the Solution Controls panel 8. Disable both the execute commands Solve-+Execute Commands 9. Continue the simulation with the same time step size for 1500 time steps to get a dynamically steady solution 10. Write the case and data files(unsteady-final cas gz and unsteady-final dat gz) te→Case&Data 10 Fluent Inc. March 12, 2008 【实例截图】
【核心代码】