Dynamic response and measurements of near-wall hot-wire system

 

Despite the availability of newer instrumentation like LDA and PIV, the hot-wire anemometer remains the workhorse of velocity measurements for many researchers and can be attributed to its relatively low cost and ease of operation. The hot-wire community, however, continued to rely on the traditional electronic ‘square-wave’ perturbation test to determine or relate the dynamic response of the system (fD) in terms of the cut-off frequency (fS) to justify the application in a turbulent flow measurement. While fD ~ fS for a wall-remote hot-wire operation, it has not been shown the case for a near-wall measurements where the complex wall effect starts to effect the hot-wire operation. Our series of work where we are one of the first to generate and impose a sufficiently high known fluctuating flow on the near-wall hot-wire/hot-film probe allow us to determine fD directly. It is found that that the marginally elevated near-wall hot wire as a velocity probe has a frequency response of O(2-3 kHz) depending on wire height in terms of wall units [Khoo et al (1998), Meas. Sci. Technol. (MST), Vol. 9, pp 751-763] which is about an order below the typical O(10-20 kHz) registered by fS. Further work for the hot-film probe as wall shear stress probe suggested that fD ~ O(1 Hz) [Chew et al (1998), MST, Vol. 9, pp 764-778] far below those obtained as O(100 kHz) by fS; this is probably why the hot-film wall shear stress probe has consistently measured a lower turbulence statistics for a shear flow in the literature. A full comparison between fS and fD, and a model provided to explain the difference observed is found in Khoo et al (1999), MST, Vol. 10, pp 152-169. Though fS and fD share similar trends of forced convection, overheating ratio and other parametric input, the absolute magnitude are vastly different! On the other hand, as the electronic perturbation test is still far simpler and easier to administer as opposed to the velocity perturbation test where one has to specially set up the required apparatus, we embarked on a different electronic-based “sine-wave” test to explore its characteristics. It is found that the latter is able to present a low frequency response feature fbulge much closer to fD (Teo et al (2001), MST, Vol. 12, pp 37-51). Therefore, one could still utilize the simpler electronic perturbation test to determine the dynamic response of the near-wall hot-wire/hot-film probe. (It is important to note that fbulge is different from the high frequency characteristic fSINE which has been established in the literature to be the same as fS.) A model for the frequency response of a near-wall hot wire is found in Khoo et al (2003), Exps Thermal Fluid Science, Vol. 27, pp 167-175. This is very different for the frequency response of a wall-remote hot wire obtained under free-stream condition with imposed known frequency [Li et al, 2007, Int. J. Heat & Fluid Flow, Vol. 28, pp 882-893]. The operation of hot wire near the wall is studied under thermally adiabatic and infinitely conducting walls and found to be affected by two (universal) dimensionless groupings relating to the wire height expressed in terms of wall units and the diameter of hot wire [Li et al, 2006, Int. J. Heat & Mass Transfer, Vol. 49, pp 905-918]; any other wall substrates lie between the two extremes of wall conductivity.

 

With the establishment of a viable means of quantifying the response of the near-wall hot wire probe to resolve the expected time scale of a turbulent shear flow, the hot wire was used to obtain the near-wall turbulence statistics of a wall bounded flow and the associated convective velocity in Taylor’s hypothesis [Part I in Khoo et al (2000), Exps Fluids, Vol. 29, pp 448-460, and Part II in Khoo et al (2001), Exps Fluids, Vol. 31, pp 494-505]. These measurements are made possible with the near-wall calibration techniques developed [Khoo et al (1996), MST, Vol. 7, pp 564-575] and the length scale effect of the hot wire resolved [Khoo et al (1997), Exps. Fluids, Vol. 22, pp 327-335]. The measurements are critical for better understanding of the near-wall behavior (since the wall acts as a source of vorticity generation and dissipation of energy) in the quest for drag reduction device(s) which seek to alter the flow very close to the wall, and for possibly establishing accurate turbulence models in the near-wall region to be used in simulation.

All these works were originated, developed and carried out within NUS. Overall, the concerted effort in near-wall hot-wire work in the past decade has established us as the leading and even perhaps the only group in the world working in this area. Supporting evidences are listed below as:

(a) Invited to present a lecture entitled, “Near-wall hot-wire response: Is square-wave voltage perturbation test adequate?” at the Mechanical Engineering Department, Auckland University on December 16, 2002.

(b) Our paper entitled, “Is square-wave electronic perturbation test adequate?” for the ASME Sixth International Thermal Anemometry Symposium in Australia on January 2001 received very supporting comment from the Chair Organizer based on the reviews received. Further invited to submit as a full paper and published in a special issue of Experimental Thermal and Fluid Science.

(c) Invited by DANTEC Dynamics to write a technical note on “Near-wall measurements with hot-wires and hot-films require special care” with Professor Dr. Durst (Friedrich-Alexander-Universitat Erlangen-Nurnburg for publication in their newsletter (Vol. 9, No. 4 - 2002) and mounted as an Application Note on their website (http://www.dantecdynamics.com).

(d) Invited by the Brazilian Association of Mechanical Sciences and Engineering to present the work on “Near-Wall Hot-Wire Measurements”, held at the Spring School on Transition and Turbulence at Rio de Janeiro, Brazil from 25-29 September 2006. This is a forum where only about a dozen foreign delegates are invited to present their findings. (All expenses are paid including the return airfare sponsored by the Brazilian National Research Council.)

(e) A project related to Near-wall hot-wire measurements for dimpled surfaces in both water and air to better understand the near-wall mechanism which may lead to drag reduction and enhanced heat transfer is funded by EADS (European Aeronautics Defense and Space), the parent of Airbus, to the amount of S$1,512,712.50 (º€ 735326) for a three year period starting end 2007. There is some matching research grant from Faculty of Engineering for the externally funded work.

 

Future work:

(a) Investigation using the pulse laser with direct perturbation heating to determine the frequency response of near-wall hot wire and its relation to fD and fS.

 

List of relevant publications

Khoo*, B C, Y T Chew and Y A Mah, "Skin friction following BLADE manipulation in a turbulent pipe flow". Experiments in Fluids, 16, no. 4/5 (1993): 274‑278. (United Kingdom).

 

Chew*, Y T, B C Khoo* and G L Li, "A time‑resolved hot‑wire shear stress probe for turbulent flow: Use of laminar flow calibration".  Experiments in Fluids, 17 (1994): 75‑83. (United Kingdom).

 

Khoo*, B C, Y T Chew and G L Li, "A new method by which to determine the dynamic response of marginally‑elevated hot‑wire anemometer probes for near‑wall velocity and wall shear stress measurements". Measurement Science and Technology, 6 (1995): 1399‑1406.  (United Kingdom).

 

Chew*, Y T, S X Shi and B C Khoo, "On the numerical near‑wall corrections of single hot‑wire measurements". International Journal of Heat and Fluid Flow, 16, no. 6 (1995): 471‑476. (United Kingdom).

 

Khoo*, B C, Y T Chew and G L Li, "Time‑resolved near‑wall hot‑wire measurements: Use of laminar flow wall correction curve and near‑wall calibration technique". Measurement Science and Technology, 7 (1996): 564‑575. (United Kingdom).

 

Khoo*, B C, Y T Chew and G L Li, "Effects of imperfect spatial resolution on turbulence measurements in the very near‑wall viscous sublayer region". Experiments in Fluids, 22 (1997): 327‑335. (United Kingdom).

 

Khoo*, B C, Y T Chew and C P Lim, "The flow between a rotating and a stationary disk: Application to near‑wall hot‑wire calibration".  Measurement Science and Technology, 9 (1998): 650‑658.  (United Kingdom).

 

Khoo*, B C, Y T Chew, C P Lim and C J Teo, "Dynamic response of hot‑wire anemometer, Part I: A marginally‑elevated hot‑wire probe for near‑wall velocity measurements". Measurement Science and Technology, 9, no. 5 (1998): 751‑763. (United Kingdom).

 

Chew*, Y T, B C Khoo*, C P Lim and C J Teo, "Dynamic response of hot‑wire anemometer.  Part II: A flush mounted hot‑wire and hot‑film probes for wall shear stress measurements". Measurement Science and Technology, 9, no. 5 (1998): 764‑778. (United Kingdom).

 

Chew*, Y T, B C Khoo* and LI G L, "An investigation of wall effects on hot‑wire measurements using a bent sublayer probe". Measurement Science and Technology, 9 (1998): 67‑85. (United Kingdom).

 

Khoo*, B C, Y T Chew*, C J Teo and C P Lim, "The dynamic response of a hot‑wire anemometer. Part III: Voltage ‑ perturbation versus velocity perturbation testing for near‑wall hot‑wire/film probes". Measurement Science and Technology, 10, no. 3 (1999): 152‑169. (United Kingdom).

 

Khoo*, B C, Y T Chew and C J Teo, "On near‑wall hot‑wire measurements". Experiments in Fluids, 29 (2000): 448‑460. (United Kingdom).

 

Khoo, B C, Y T Chew and C J Teo, "Near‑wall hot‑wire measurement. Part 2: Turbulence time scale, convective velocity and spectra in the viscous sublayer". Experiments in Fluids, 31 (2001): 494‑505. (United Kingdom).

 

Teo, C J, B C Khoo* and Y T Chew*, "Dynamic response of a hot‑wire anemometer. Part IV: Sine‑wave voltage perturbation test for near‑wall hot‑wire/film probe and presence of low‑high frequency response". Measurement Science and Technology, 12 (2001): 37‑51. (United Kingdom).

 

Khoo*, B C, Y T Chew and C J Teo, "A model for the frequency response of a near‑wall hot‑wire: Velocity perturbation and sine‑wave voltage perturbation tests". Experimental Thermal and Fluid Science, 27, no. 2 (2003): 167‑175. (United States).

 

Li, W Z, B C Khoo* and D Xu, "The thermal characteristics of a hot wire in a near‑wall flow". International Journal of Heat & Mass Transfer, 49 (2005): 905‑918. (United Kingdom).

 

Li, W.Z., B C Khoo* and D. Xu, "The thermal characteristics of a hot wire in a fluctuating free stream flow". International Journal of Heat and Fluid Flow, 28 (2007): 882-893. (United Kingdom).