The Influence of Adjacent Sweeping Jets
3.2 Experimental Setup
To answer some of these questions a bench-top model was built where the distance between two adjacent sweeping jet actuators could easily be changed. The experimental setup is shown in Fig. 3.1a. Actuators can be freely moved along a slot to change their respective spacing and more
(a)Bench-top model to study the influence of adjacent jets on each other depending on the separation distance. The swee- ping jets actuators are colored in blue.
(b)Single sweeping jet ac- tuator that was used for the experiment.
Figure 3.1:Bench-top model setup.
actuators can be added if necessary; experiments with three or more actuators are possible. However, the data shown here will only be used to discuss the case of two adjacent sweeping jet actuators in quiescent ambient air. The absence of a freestream flow ensures that solely the interaction between the two adjacent sweeping jets depending on their separation distance can be examined. A schematic drawing of the sweeping jet actuator design is shown in Fig. 3.1b, where the actuator height is 0.050 inches and the nozzle width is 0.100 inches. To ensure identical pressure, each actuator has a separate connection to a manifold. To observe their respective interaction, the sweeping jet actuators were placed in the same Schlieren setup that was used to acquire the images inChapter 2. For this experiment, the IDT MotionPro Y7 high-speed camera acquired images at a frame rate of 9000 Hz.
Due to the physical size of the setup, the separation distance of the two actuators was limited to 1.25 inches to observe both of them simultaneously. For higher separation distances, the second actuator was out-of-view of the camera. The analysis is then solely based on comparison with the reference case of a single sweeping jet actuator.
Figure 3.2:Definition of the deflection angle for the top (θ1) and bottom (θ2) sweeping jet actuator. The spacing between the actuators is labeled withd.
It was believed, that any influence the actuators exhibit would translate into a position or frequency adjustments of one or even both jets to accommodate the presence of the other. To put “position”
into an actual quantitative value, it was decided to record the jet deflection angles of the actuators as defined inFig. 3.2. To extract the angle deflectionsθ1andθ2from the images an algorithm was written to track the jets and calculate their respective deflection angles. The images were analyzed using an edge detection method introduced byCanny, 1986. The method calculates the gradient of
(a)Raw image (b)Processed image
(c)Overlay
Figure 3.3:Detection of the jets using the Canny edge detection method.
an intensity image using the derivative of a Gaussian filter. This method applies two thresholds to the gradient: a high threshold for low edge sensitivity and a low threshold for high edge sensitivity.
It starts with the low sensitivity result and then grows it to include connected edge pixels from the high sensitivity result. This helps fill in gaps in the detected edges and makes the Canny method less likely to be fooled by noise compared to other methods and more likely to detect true weak edges.
InFig. 3.3one can see a comparison between a raw image, the processed version, and an overlay of both. It’s evident that this method is fully capable of tracking the jets without much difficulty.
However, the main interest is acquiring the deflection angle and not tracking the jets themselves. A simple way to do so is by calculating the center of mass of the tracked pixels of the processed image (3.3b). This is more difficult than it sounds because the large wake region that has already mixed well with the surrounding air lags considerably behind the actual jet deflection angle. Fig. 3.4a
(a)Full detection area with inaccurate center of mass cal- culation (dot).
(b)Detection area limited to a narrow band right after the jet.
Figure 3.4:Reduction of the detection area significantly improves the accuracy of the center of mass calcu- lation (dots).
exemplifies this problem: The large wake region of the jet lags behind the actual exit jet and causes the center of mass calculation to be inaccurate (dot in the images). Fortunately, by limiting the tracking area to a small band closer to the actuator exit, where the jet has not started its mixing and wake interactions, this can be resolved. Fig. 3.4buses a much narrower band and the accuracy of the calculated center of mass is significantly improved. By knowing the location of the jet, simple trigonometric calculations lead to the sought-after deflection angle.
The first images taken showed that two adjacent sweeping jet actuators went through cycles with three distinguishable states: parallel-sync, crossed-sync, and async. These states are defined as:
parallel-sync: θ1 = θ2, (3.1)
cross-sync: θ1 =−θ2, (3.2)
async: θ1 , θ2. (3.3)
Parallel-sync is a parallel motion of the jet with respect to each other, while cross-sync means the jets are mirrored along the horizontal center line resulting in a criss-cross behavior. Async describes all the other jet orientations where the jets are in a random orientation to each other. The two in-sync states are illustrated inFig. 3.5.
(a)Parallel-sync:θ1=θ2 (b)Cross-sync:θ1=−θ2 Figure 3.5:Illustration of the two in-sync states of the jets.