Langley Research Center Turbulence Modeling Resource

Exp: CFDVAL2004 Case 2 Frequently Asked Questions

Return to: CFDVAL2004 Case 2 - Intro Page

Q:How is the "phase angle 0 deg" defined in the experiment?

A:The phase angle in the experiment is based on the input voltage that drives the actuator (see the file "Driver BCs from experiment" under Case 2 Details and Submission Guidelines). There is naturally a phase lag between the voltage input and the actual velocity that emanates from the orifice opening. It ends up that the peak (maximum) velocity out of the orifice occurs somewhere near 110-120 deg, as shown in the first figure on that webpage (plot of OverExit.dat). Ultimately, the goal is to align CFD's phase to approximately match the experiment as best as possible over the course of the entire 360 degrees of phase at the orifice exit shown in this figure. We have tried to choose criteria for determining phase that approximates experiment at this location AND is specific enough so that different CFD solutions can be meaningfully compared. This method is described near the bottom of the above-mentioned webpage. Although participants are given some latitude to determine phase as appropriate, we encourage everyone to use the prescribed method for consistency, if possible.

Q:What is the nominal tunnel "freestream" turbulence level in the 15 x 9.8 inch tunnel?

A:In previous testing, the Tu level was measured at approximately 0.13% at M=0.1.

Q:What is the volume of the cavity?

A:The nominal as-built volume of the cavity in Case 2 is approximately 22674 mm³ (computed from the 3-D structured grid). This includes the volume of the orifice up to the tunnel floor. However, note that when the wind tunnel is on and when the moveable piston is operating, the neutral position of the plate moves up by about 1.1 mm (see brief discussion under Case 2 Details and Submission Guidelines). The amount that the volume is reduced because of this movement is difficult to calculate because the piston wall (bottom floor of the cavity) is part flexible membrane and part solid piston face. An estimate is a reduction of roughly 4600 mm³.

Q:What is the excitation voltage delivered to the actuator?

A:5.17 Volts rms.

Q:Can you describe the piston arrangement for Case 2?

A:The piston consists of a solid plate mounted on a flexible membrane; it is driven electro-mechanically. As shown under Case 2 Geometry, the flexible membrane face is approximately 76 mm by 76.3 mm in size, and the solid plate, crudely centered on the membrane, is roughly 51 mm by 50.4 mm in size. The solid plate is 2 mm thick. At rest, the top of the piston face is located 2.78 mm below the top inside of the driver cavity. However, as mentioned above (under "What is the volume of the cavity?") and in the brief discussion under Case 2 Details and Submission Guidelines, when the wind tunnel is on and when the moveable piston is operating, the neutral position of the plate moves up by about 1.1 mm.

Return to: CFDVAL2004 Case 2 - Intro Page

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Last Updated: 05/15/2021