Langley Research Center Workshop CFD Validation of Synthetic Jets and Turbulent Separation Control

Case 3 (Introduction)

Case 3 is a Wall-mounted Glauert-Goldschmied type body, geometrically similar to that employed by Seifert & Pack (2002). Full details of the current case are given in Greenblatt et al (2004, 2005) and Naughton et al (2004). The model is mounted between two glass endplate frames and both leading edge and trailing edges are faired smoothly with a wind tunnel splitter plate. This is a nominally two-dimensional experiment, although there are side-wall effects (3-D flow) near the end-plates. The tunnel dimensions at the test section are 28 inches wide by 20 inches high, but the hump model is mounted on a splitter plate (0.5 inches thick), yielding a nominal test section height of 15.032 inches (distance from the splitter plate to the top wall). The splitter plate extends 76.188 inches upstream of the model's leading edge. Also, 44.437 inches downstream of the model's leading edge, the splitter plate is equipped with a flap (3.75 inches long), which is deflected up during the experiment in order to control the air flow beneath the splitter plate. This control affects the stagnation point at the leading edge of the splitter plate, avoiding massive separation in that region.

The characteristic reference length of the model is defined here as the length of the bump on the wall, 16.536 inches. (In Seifert & Pack's original work on the hump model, the reference length was defined as the airfoil chord length of 7.874 inches (x = 0.0 to x = 7.874 inches). Their leading edge was then faired smoothly into the wall from x = -.3937 to x = .3937 inches; however this additional length ahead of x = 0.0 was not accounted for in their definition of the reference length. For the current experiment it was felt to be more straightforward to use the actual bump length as the reference length. As a result of this, the current scaled (nondimensional) coordinates of the overall body shape are slightly different from those of Seifert & Pack. A simple rescaling operation can recover it.)

The model itself is 23 inches wide between the endplates at both sides (each endplate is approximately 9.25 inches high, 34 inches long, and 0.5 inches thick with an elliptical-shaped leading edge). The model is 2.116 inches high at its maximum thickness point. Both uncontrolled (baseline) and controlled flow scenarios are considered under the conditions of M = 0.1 and Re somewhat less than 1 million per chord. The tunnel medium is air at sea level. The model experiences a fully-developed turbulent boundary layer whose delta (thickness) at the leading edge of the model is between 1 - 2 inches. The boundary layer is subjected to a favorable pressure gradient over the front convex portion of the body and separates over a relatively short concave section in the aft part of the body. A slot opening of 0.00187*chord (chord=0.4200 m) at approximately the 65% chord station on the model extends across the entire span of the hump ("slot opening" here means vertical opening below the lip - see figure below).

Flow control is supplied by means of the two-dimensional slot across the span, immediately upstream of the concave surface. One type of control uses steady suction, which is driven by a suction pump with the mass flow monitored. The following two figures show a schematic of the hump model and a sample 2-D CFD result.

For the oscillatory test, separation control is performed using zero efflux oscillatory blowing introduced from the spanwise slot, where careful attention is paid to maintaining slot two-dimensionality. This is achieved by means of a rigid piston that spans the model. The piston is secured to a flange by means of a flexible membrane and the flange is bolted to the base of the plenum. The piston is driven externally by six voice-coil-based actuator modules (Kiedaisch, Nagib and Associates, IIT), generating slot velocities up to 80m/s at frequencies ranging from 60Hz to 500Hz. A schematic view of the assembly from the side is shown in fig. 1 below; a view from the underside of the model, with three modules removed for purposes of illustrating the piston and membrane, is show in fig. 2. A photograph of the underside of the model, prior to tunnel installation, is shown in fig. 3. The slot-flow is calibrated and characterized for both tunnel flow-off (quiescent) and flow-on (non-quiescent) conditions, using hot-wire anemometry, throat dynamic pressure measurements and two-dimensional PIV.

Fig. 1

Fig. 2

Fig. 3

Note that the two endplate frames create blockage which affects all the measurements for this case. See "Experimantalist's Note 3", in the Experimental Data link below. Also, one of the questions in the FAQS link below addresses this issue.

References:

Greenblatt, D., Paschal, K. B., Yao, C.-S., Harris, J., Schaeffler, N. W., Washburn, A. E., "A Separation Control CFD Validation Test Case, Part 1: Baseline and Steady Suction," AIAA Journal, Vol. 44, No. 12, 2006, pp. 2820-2830.

Greenblatt, D., Paschal, K. B., Yao, C.-S., Harris, J., "A Separation Control CFD Validation Test Case, Part 2: Zero Efflux Oscillatory Blowing," AIAA Journal, Vol. 44, No. 12, 2006, pp. 2831-2845.

Naughton, J. W., Viken, S. A., Greenblatt, D., "Skin-Friction Measurements on the NASA Hump Model," AIAA Journal, Vol. 44, No. 6, 2006, pp. 1255-1265.

Seifert, A. and Pack, L. G., "Active Flow Separation Control on Wall-Mounted Hump at High Reynolds Numbers," AIAA Journal, Vol. 40, No. 7, July 2002.

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