Dimensionless parameters such as the Reynolds number are of uttermost importance in fluid dynamics as they appear in the dimensionless conservation equations. The precise material parameters are not important as long as the relevant dimensionless numbers have a similar magnitude (law of similarity). This allows us to conduct experiments with properly designed smaller systems. For the Reynolds number for instance this means that only rescaling a model and thus the characteristic length is not sufficient to have similar turbulence behaviour such as similar turbulent structures. Instead you have to adapt either characteristic velocity or viscosity or both as well. If you additionally demand other dimensionless numbers to be similar as well, such as the Mach number, you soon have very limited options for conducting model experiments. Most of these dimensionless numbers are a ratio of certain opposed properties and allow to estimate which are the dominating effects in a particular situation.

The Reynolds number

$$ Re := \frac{ U L }{ \nu } = \frac{\rho \, U \, L}{\mu} \label{1}\tag{1}$$

appears in the conservation equations several times

$$\frac{\partial \rho^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} \frac{\partial (\rho^* u_j^* )}{\partial x_j^* }=0 \label{2}\tag{2}$$

$$\rho^* \frac{\partial u_i^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial u_i^*}{\partial x_j^*} = - \frac{\partial p^*}{ \partial x_i^* } + \frac{1}{Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial \tau_{ij}^*}{\partial x_j^* } + \frac{1}{Fr^2} g_i^* \label{3}\tag{3}$$

$$\rho^* \frac{\partial T^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial T^*}{\partial x_j^*} = Ec \left( \frac{\partial p^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial p^*}{\partial x_j^*} \right) + \frac{1}{Pr Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial}{\partial x_j^*} \left( \frac{\partial T^*}{\partial x_j^*} \right) + \frac{Ec}{Re} \sum\limits_{i, j \in \mathcal{D}} \tau _{ij}^* \frac{\partial u_i^*}{\partial x_j^*} \label{4}\tag{4}$$

It appears in front of the dissipative terms in the momentum \ref{3} and energy equation \ref{4}. Dissipative terms act similar to dampers, they dissipate energy: they take macroscopic energy and convert it to thermal energy and thus smooth out gradients, they equalise. The corresonding damping coefficient is given by the dynamic viscosity $\mu$ with appears in the denominator of equation \ref{1}. For large Reynolds number the dissipative effects can be neglected and we end up with the Euler's equations for inviscid flow.

Additionally you can see that the numerator takes the form of some sort of momentum $\propto \rho U$. So our basic idea is to cast this into a form of the two opposed properties of inertia and dissipation.

Let's take Newton's first law, and rewrite it using characteristic measures

$$ F_{in} = m a \approx \underbrace{ \rho A L }_{m = \rho V} \frac{L}{T^2} = \rho A U^2 \label{5}\tag{5}$$

on the other hand someone could define a viscous force in a similar manner:

$$ F_{\mu} \propto \frac{\mu A \Delta U}{\Delta L} \approx \frac{\mu A U}{L} \label{6}\tag{6}$$

as the shear-rate $S_{ij}$ is connected to viscous stresses $\tau_{ij}$

$$ \frac{F_{\mu}}{A} = \tau_{ij} = 2 \mu S_{ij} - \frac{2}{3} \mu \sum\limits_{k \in \mathcal{D}} S_{kk} \delta_{ij} \label{7}\tag{7}$$

where

$$S_{ij} := \frac{1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right). \label{8}\tag{8}$$

We can see that dividing these two forces - $F_{in}$ and $F_{\mu}$ - results in the Reynolds number and thus

$$ Re \propto \frac{\text{inertial forces}}{\text{viscous damping forces}}. \label{9}\tag{9}$$

It basically is a measure wherever our floe will be dominated by inertia, become turbulent with small whirls and large local gradients or smooth out quickly and be laminar as for low Reynolds number flow (in particular Stokes' flow $Re \lesssim \mathcal{O}(1)$).

In my opinion thinking in terms of a Reynolds velocity is not very helpful instead always think for dimensionless numbers in term of ratios of forces and diffusivities. In the end we are only trying to estimate orders of magnitude.

Dimensionless parameters such as the Reynolds number are of uttermost importance in fluid dynamics as they appear in the dimensionless conservation equations. The precise material parameters are not important as long as the relevant dimensionless numbers have a similar magnitude (law of similarity). This allows us to conduct experiments with properly designed smaller systems. For the Reynolds number for instance this means that only rescaling a model and thus the characteristic length is not sufficient to have similar turbulence behaviour such as similar turbulent structures. Instead you have to adapt either characteristic velocity or viscosity or both as well. If you additionally demand other dimensionless numbers to be similar as well, such as the Mach number, you soon have very limited options for conducting model experiments. Most of these dimensionless numbers are a ratio of certain opposed properties and allow to estimate which are the dominating effects in a particular situation.

The Reynolds number

$$ Re := \frac{ U L }{ \nu } = \frac{\rho \, U \, L}{\mu} \label{1}\tag{1}$$

appears in the conservation equations several times

$$\frac{\partial \rho^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} \frac{\partial (\rho^* u_j^* )}{\partial x_j^* }=0 \label{2}\tag{2}$$

$$\rho^* \frac{\partial u_i^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial u_i^*}{\partial x_j^*} = - \frac{\partial p^*}{ \partial x_i^* } + \frac{1}{Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial \tau_{ij}^*}{\partial x_j^* } + \frac{1}{Fr^2} g_i^* \label{3}\tag{3}$$

$$\rho^* \frac{\partial T^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial T^*}{\partial x_j^*} = Ec \left( \frac{\partial p^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial p^*}{\partial x_j^*} \right) + \frac{1}{Pr Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial}{\partial x_j^*} \left( \frac{\partial T^*}{\partial x_j^*} \right) + \frac{Ec}{Re} \sum\limits_{i, j \in \mathcal{D}} \tau _{ij}^* \frac{\partial u_i^*}{\partial x_j^*} \label{4}\tag{4}$$

It appears in front of the diffusive terms in the momentum \ref{3} and energy equation \ref{4}. Diffusive/dissipative terms act similar to dampers, they dissipate energy: they take macroscopic energy and convert it to thermal energy and thus smooth out gradients, they equalise. The corresponding damping coefficient is given by the dynamic viscosity $\mu$ with appears in the denominator of equation \ref{1}. For large Reynolds number the dissipative effects can be neglected and we end up with the Euler's equations for inviscid flow.

Additionally you can see that the numerator takes the form of some sort of momentum $\propto \rho U$. So our basic idea is to cast this into a form of the two opposed properties of inertia and dissipation.

Let's take Newton's first law, and rewrite it using characteristic measures

$$ F_{in} = m a \approx \underbrace{ \rho A L }_{m = \rho V} \frac{L}{T^2} = \rho A U^2 \label{5}\tag{5}$$

on the other hand someone could define a viscous force in a similar manner:

$$ F_{\mu} \propto \frac{\mu A \Delta U}{\Delta L} \approx \frac{\mu A U}{L} \label{6}\tag{6}$$

as the shear-rate $S_{ij}$ is connected to viscous stresses $\tau_{ij}$

$$ \frac{F_{\mu}}{A} = \tau_{ij} = 2 \mu S_{ij} - \frac{2}{3} \mu \sum\limits_{k \in \mathcal{D}} S_{kk} \delta_{ij} \label{7}\tag{7}$$

where

$$S_{ij} := \frac{1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right). \label{8}\tag{8}$$

We can see that dividing these two forces - $F_{in}$ and $F_{\mu}$ - results in the Reynolds number and thus

$$ Re \propto \frac{\text{inertial forces}}{\text{viscous damping forces}}. \label{9}\tag{9}$$

It basically is a measure wherever our flow will be dominated by inertia, become turbulent with small whirls and large local gradients or smooth out quickly and be laminar as for low Reynolds number flow (in particular Stokes' flow $Re \lesssim \mathcal{O}(1)$).

In my opinion thinking in terms of a "Reynolds speed" is not particularly useful: You could also introduce a "Reynolds length scale" $l_{Re} = \frac{\mu}{\rho U}$ or a "Reynolds viscosity" $\mu_{Re} = \rho U L$ or a "Reynolds density" $\rho_{Re} = \frac{\mu}{U L}$ and try to find a meaning for them. Instead always think for dimensionless numbers in term of ratios of forces and diffusivities. In the end we are only trying to estimate orders of magnitude.

Dimensionless parameters such as the Reynolds number are of uttermost importance in fluid dynamics as they appear in the dimensionless conservation equations. The precise material parameters are not important as long as the relevant dimensionless numbers have a similar magnitude (law of similarity). This allows us to conduct experiments with properly designed smaller systems. For the Reynolds number for instance this means that only rescaling a model and thus the characteristic length is not sufficient to have similar turbulence behaviour such as similar turbulent structures. Instead you have to adapt either characteristic velocity or viscosity or both as well. If you additionally demand other dimensionless numbers to be similar as well, such as the Mach number, you soon have very limited options for conducting model experiments. Most of these dimensionless numbers are a ratio of certain opposed properties and allow to estimate which are the dominating effects in a particular situation.

The Reynolds number

$$ Re := \frac{ U L }{ \nu } = \frac{\rho \, U \, L}{\mu} \label{1}\tag{1}$$

appears in the conservation equations several times

$$\frac{\partial \rho^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} \frac{\partial (\rho^* u_j^* )}{\partial x_j^* }=0 \label{2}\tag{2}$$

$$\rho^* \frac{\partial u_i^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial u_i^*}{\partial x_j^*} = - \frac{\partial p^*}{ \partial x_i^* } + \frac{1}{Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial \tau_{ij}^*}{\partial x_j^* } + \frac{1}{Fr^2} g_i^* \label{3}\tag{3}$$

$$\rho^* \frac{\partial T^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial T^*}{\partial x_j^*} = Ec \left( \frac{\partial p^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial p^*}{\partial x_j^*} \right) + \frac{1}{Pr Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial}{\partial x_j^*} \left( \frac{\partial T^*}{\partial x_j^*} \right) + \frac{Ec}{Re} \sum\limits_{i, j \in \mathcal{D}} \tau _{ij}^* \frac{\partial u_i^*}{\partial x_j^*} \label{4}\tag{4}$$

It appears in front of the dissipative terms in the momentum \ref{3} and energy equation \ref{4}. Dissipative terms act similar to dampers, they dissipate energy: they take macroscopic energy and convert it to thermal energy and thus smooth out gradients, they equalise. The corresonding damping coefficient is given by the dynamic viscosity $\mu$ with appears in the denominator of equation \ref{1}. For large Reynolds number the dissipative effects can be neglected and we end up with the Euler's equations for inviscid flow.

Additionally you can see that the numerator takes the form of some sort of momentum $\propto \rho U$. So our basic idea is to cast this into a form of the two opposed properties of inertia and dissipation.

Let's take Newton's first law, and rewrite it using characteristic measures

$$ F_{in} = m a \approx \underbrace{ \rho A L }_{m = \rho V} \frac{L}{T^2} = \rho A U^2 \label{5}\tag{5}$$

on the other hand someone could define a viscous force in a similar manner:

$$ F_{\mu} \propto \frac{\mu A \Delta U}{\Delta L} \approx \frac{\mu A U}{L} \label{6}\tag{6}$$

as the shear-rate $S_{ij}$ is connected to viscous stresses $\tau_{ij}$

$$ \tau_{ij} = 2 \mu S_{ij} - \frac{2}{3} \mu \sum\limits_{k \in \mathcal{D}} S_{kk} \delta_{ij} \label{7}\tag{7}$$

where

$$S_{ij} := \frac{1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right). \label{8}\tag{8}$$

We can see that dividing these two forces - $F_{in}$ and $F_{\mu}$ - results in the Reynolds number and thus

$$ Re \propto \frac{\text{inertial forces}}{\text{viscous damping forces}}. \label{9}\tag{9}$$

In my opinion thinking in terms of a Reynolds velocity is not very helpful instead always think for dimensionless numbers in term of ratios of forces and diffusivities.

Dimensionless parameters such as the Reynolds number are of uttermost importance in fluid dynamics as they appear in the dimensionless conservation equations. The precise material parameters are not important as long as the relevant dimensionless numbers have a similar magnitude (law of similarity). This allows us to conduct experiments with properly designed smaller systems. For the Reynolds number for instance this means that only rescaling a model and thus the characteristic length is not sufficient to have similar turbulence behaviour such as similar turbulent structures. Instead you have to adapt either characteristic velocity or viscosity or both as well. If you additionally demand other dimensionless numbers to be similar as well, such as the Mach number, you soon have very limited options for conducting model experiments. Most of these dimensionless numbers are a ratio of certain opposed properties and allow to estimate which are the dominating effects in a particular situation.

The Reynolds number

$$ Re := \frac{ U L }{ \nu } = \frac{\rho \, U \, L}{\mu} \label{1}\tag{1}$$

appears in the conservation equations several times

$$\frac{\partial \rho^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} \frac{\partial (\rho^* u_j^* )}{\partial x_j^* }=0 \label{2}\tag{2}$$

$$\rho^* \frac{\partial u_i^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial u_i^*}{\partial x_j^*} = - \frac{\partial p^*}{ \partial x_i^* } + \frac{1}{Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial \tau_{ij}^*}{\partial x_j^* } + \frac{1}{Fr^2} g_i^* \label{3}\tag{3}$$

$$\rho^* \frac{\partial T^*}{\partial t^*} + \rho^* \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial T^*}{\partial x_j^*} = Ec \left( \frac{\partial p^*}{\partial t^*} + \sum\limits_{j \in \mathcal{D}} u_j^* \frac{\partial p^*}{\partial x_j^*} \right) + \frac{1}{Pr Re} \sum\limits_{j \in \mathcal{D}} \frac{\partial}{\partial x_j^*} \left( \frac{\partial T^*}{\partial x_j^*} \right) + \frac{Ec}{Re} \sum\limits_{i, j \in \mathcal{D}} \tau _{ij}^* \frac{\partial u_i^*}{\partial x_j^*} \label{4}\tag{4}$$

It appears in front of the dissipative terms in the momentum \ref{3} and energy equation \ref{4}. Dissipative terms act similar to dampers, they dissipate energy: they take macroscopic energy and convert it to thermal energy and thus smooth out gradients, they equalise. The corresonding damping coefficient is given by the dynamic viscosity $\mu$ with appears in the denominator of equation \ref{1}. For large Reynolds number the dissipative effects can be neglected and we end up with the Euler's equations for inviscid flow.

Additionally you can see that the numerator takes the form of some sort of momentum $\propto \rho U$. So our basic idea is to cast this into a form of the two opposed properties of inertia and dissipation.

Let's take Newton's first law, and rewrite it using characteristic measures

$$ F_{in} = m a \approx \underbrace{ \rho A L }_{m = \rho V} \frac{L}{T^2} = \rho A U^2 \label{5}\tag{5}$$

on the other hand someone could define a viscous force in a similar manner:

$$ F_{\mu} \propto \frac{\mu A \Delta U}{\Delta L} \approx \frac{\mu A U}{L} \label{6}\tag{6}$$

as the shear-rate $S_{ij}$ is connected to viscous stresses $\tau_{ij}$

$$ \frac{F_{\mu}}{A} = \tau_{ij} = 2 \mu S_{ij} - \frac{2}{3} \mu \sum\limits_{k \in \mathcal{D}} S_{kk} \delta_{ij} \label{7}\tag{7}$$

where

$$S_{ij} := \frac{1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right). \label{8}\tag{8}$$

We can see that dividing these two forces - $F_{in}$ and $F_{\mu}$ - results in the Reynolds number and thus

$$ Re \propto \frac{\text{inertial forces}}{\text{viscous damping forces}}. \label{9}\tag{9}$$

It basically is a measure wherever our floe will be dominated by inertia, become turbulent with small whirls and large local gradients or smooth out quickly and be laminar as for low Reynolds number flow (in particular Stokes' flow $Re \lesssim \mathcal{O}(1)$).

In my opinion thinking in terms of a Reynolds velocity is not very helpful instead always think for dimensionless numbers in term of ratios of forces and diffusivities. In the end we are only trying to estimate orders of magnitude.