4. EXPERIMENTS AT MIR FLOW FACILITY
4.2 Bypass Transitional Flow Measurements of a Flat Plate
4.2.2 Benchmark Studies
To calibrate the MIR system and flat plate study, two simplistic studies were performed in the oil tunnel to benchmark against readily available flat plate data: (1) a zero pressure gradient (ZPG) without the
turbulence generator (TG) installed (as shown in Figure 30), and (2) ZPG with the TG installed. The ZPG case was achieved by assuming the tunnel walls were a “negligible” favorable pressure gradient. The former was found to be in direct comparison to the Blasius profile, holding that the case was in fact laminar down the length of the plate. The later benchmark of ZPG with a TG was mainly used as a connection between the laminar flow found in the ZPG without a TG and the fully installed APG with a TG. Both the ZPG cases (with and without a TG) were helpful in determining transitional behavior in the overall experiment. PIV data very near walls break down and overestimate the flow in this region, due mainly to the discrete size of the IWs always measuring some flow within.
Figure 30. Experimental data achieved in the center window of the MIR during the ZPG without a TG installed.
4.2.3 Test Plan
Planar PIV was used to capture the flow over the flat plate. Two FOVs were used: large scale FOV (at approximately 6 in.) and a mezzo FOV (at approximately 0.5 in.). The image plane was located at the centerline of the tunnel/plate with the normal axis in the tunnel/plate span wise direction. The camera was set up perpendicular to the image plane just above the flat plate, with the bottom of the image slightly viewing the top surface of the flat plate. As much data as could be acquired down the length of the plate within the three windows of the tunnel was recorded, with the camera traversing downstream from the leading edge of the plate. The tunnel’s main axial pump was ramped up to maximum capacity allowed by the MIR specification sheet.
4,000 image pairs were recorded at each location in each FOV (both large and mezzo). This was determined from the Uzol study mentioned above as the measurements from the image sets appeared to converge around 4,000 image pairs. Also, to further improve the wall normal gradient measurements, the time between image pairs in the mezzo FOV was varied based on local flow conditions to utilize two different time stamps. One was curtailed as a “long” dt while the other was a “short” dt; in this sense, the longer dt could resolve the slower flow near the wall (even down to the zero velocity at the wall), and the short dt better resolved the fluid flow that bridges the large FOV data sets to the mezzo FOV data sets, as shown in Figure 31. In contrast, only every other location was recorded, as the wall normal gradient was found to be more crucial in accurate measurement than to have a streamwise continuous measurement. This allowed for saving space on the hard drives for the important measurements.
0 1 2 3 4 5 6 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 Ș u/U Blasius Middle window - No TG
Figure 31. APG case having large FOV and mezzo FOV (by varying the dt between camera images).
4.2.4 Flow Rate Analysis
The quantity of data obtained from this study is staggering. Just beyond 10 terabytes of data were recorded at the MIR facility, and analyzing its entirety is currently underway. Some preliminary results have been found to show reasonable behavior of bypass transition within the confines of the oil tunnel.
The freestream conditions of the ZPG with the TG (see Figure 32) show the initial freestream turbulence intensity as a percent of the fluctuations to the freestream streamwise velocity component to start out at ~9% near the leading edge of the plate and decrease exponentially to ~4.5%. Ideally, the freestream flow is uniform, but due to the tunneling effects of the MIR test section, there is a slight increase in freestream velocity. The Reynolds number, based on x position and boundary layer thickness, grows along the length of the plate.
To further understand the flow phenomenon, and making sure to aquire transistional flow regime within the tunnel, experimental results were compared to a Direct Numerical Simulation (DNS) data set. Figure 33 shows both the experimental ZPG and the DNS ZPG as skin friction coefficient (Cf) versus plate location. It is the main focus of this study to observe the minimum value of Cf, as it immediately follows the
transitional region of the flow. Also, on this graph is a plot of the experimental APG and its DNS comparison, APG4 (where the 4 in this label represents the amount of lamda = 0.04, lamda being the
dimensionless pressure gradient parameter Ȝ = į/IJw*dp/dx). The third window of data in this set is still under
investigation, but the minimum in Cf can be observed slightly downstream from that of the APG4 Cf minimum. This is consistent, as the experimental APG has a slightly lower pressure gradient than what was modeled in the DNS.
Figure 32. ZPG with TG installed preliminary results: (a) the freestream turbulence intensity versus plate location, (b) Reynolds number based on x versus plate location, (c) freestream streamwise velocity component versus plate location, and (d) boundary layer thickness versus plate location.
Figure 33. Comparison of direct numerical simulation with experimental results from the MIR flat plate experiment.
a)
d) c)
4.2.5 Conclusions
A flat plate was installed into the MIR oil tunnel to quantify entropy generation within the bypass transitional region of wall-bounded flow. The study is still under investigation, but preliminary results have shown that transitional flow has been achieved within the confines of the tunnel test section. This was accomplished mainly by the use of a TG and an APG (diverging plate) that feeds the boundary layer and brings the transition to turbulence closer to the leading edge.