Hibernating Turbulence

Recent Direct Numerical Simulations (DNS) of low Reynolds number (Re=3600) turbulent channel flow of dilute polymer solutions have discovered - even in the Newtonian limit - intervals of so-called "hibernating" turbulence (Xi and Graham (2010a)). In this state many features of fully drag-reduced flow are observed (i.e. features associated with Virk's Maximum Drag Reduction Asymptote MDRA Virk (1975)); weak streamwise structures, reduced Reynolds shear stresses and, as a consequence, enhanced mass flux (at constant driving pressure) i.e. drag reduction occurs. The aim of this project is to search for experimental verification of this hibernating turbulence and extract the coherent structures within it over a range of Reynolds numbers. Ultimately, enhanced knowledge of hibernation can pave the way for novel flow-control strategies to maintain this hibernating state and therefore substantially reduce skin friction drag.

It is anticipated that the fundamental understanding gained from this project will subsequently be applied to a follow-on proposal to implement a number of possible flow control concepts. The intention here would be to devise strategies to encourage hibernating turbulence without recourse to polymer addition. If hibernating turbulence can be encouraged (at the expense of active turbulence) then this opens the door to numerous practical applications for skin-friction reduction and huge potential impact. Reduction of skin-friction drag has been a major goal for many decades and there are obviously huge and unlimited commercial potentials: the Alaskan pipe line is often cited as a major commercial example of reducing pumping costs but decreasing drag around ships and or submarines and improving efficiency in district heating and cooling systems or geothermal systems are a few other examples of vast commercial exploitability. In Japan, for example, district heating and cooling systems are becoming increasing widely used and there are now over 100 installations [Ref 6 in CfS]. The benefits of comparable reduction in frictional drag without recourse to polymer addition - through flow control to maintain hibernating turbulence for example - clearly offer an even greater avenue for net energy savings. An additional benefit of the increased understanding that this proposal will produce is that it may offer different routes for active control techniques. We anticipate that the fundamental understanding gained from this future project will subsequently be applied to practical engineering and we would look to the Knowledge Transfer Partnership (KTP) scheme in collaboration with industry, in the medium future (in 7-10 years time), to produce practical applications. As turbulence control and reduction has been a major scientific goal for many over the past 50 years or so, such ideas are at the high end of the high risk/high return spectrum of research. Without the fundamental experiments proposed in this study, future development in this exciting area (i.e. this recently discovered hibernating turbulence state) will be more limited.