Hydrodynamics in Porous Media

Presentation Transcript

Hydrodynamics in Porous Media We will cover: How liquids react to neighborhood potential inclinations (Darcy's Law) Add the preservation of mass to acquire Richard's condition

Darcy's Law for immersed media In 1856 Darcy enlisted to make sense of the water supply to the town's focal wellspring. Tentatively found that flux of water permeable media could be communicated as the result of the imperviousness to stream which described the media, and strengths acting to "push" the liquid through the media. Q - The rate of stream (L 3/T) as the volume of water went through a segment for every unit time. h i - The liquid potential in the media at position i, gauged in standing head proportionate. Under immersed conditions this is made out of gravitational potential (height), and static weight potential (L: drive per unit territory separated by  g). K - The pressure driven conductivity of the media. The proportionality between particular flux and forced angle for a given medium (L/T). L - The length of media through which stream passes (L). A - The cross-sectional range of the segment (L 2 ).

Darcy's Law Darcy then watched that the stream of water in a vertical section was very much portrayed by the condition Darcy's demeanor is composed in a general shape for isotropic media as q is the particular flux vector (L/T; volume of water per unit region per unit time), K is the soaked pressure driven conductivity tensor (second rank) of the media (L/T), and  H is the slope in water driven head (dimensionless)

Aside on analytics ... What is this up-side-down triangle about? The "dell" administrator: short hand for 3-d subordinate The aftereffect of "working" on a scalar capacity (like potential) with  is the slant of the capacity  F focuses straightforwardly towards the steepest bearing of up slope with a length corresponding to the incline of the slope. Later we'll utilize  •F. The speck just instructs us to take the dell and ascertain the spot result of that and the capacity F (which should be a vector for this to bode well). "dell-speck F" is the "dissimilarity" of F. In the event that F were neighborhood flux (with greatness and heading),  •F would be the measure of water leaving the point x,y,z. This is a scalar outcome!  F takes a scalar capacity F and gives a vector incline  •F utilizes a vector work F and gives a scalar outcome.

Now, about those parameters... Inclination in head is dimensionless, being length per length Q = Aq Q has units volume per unit time Specific flux, q, has units of length per time, or speed. For vertical stream: speed at which the tallness of a lake of liquid would drop CAREFUL: q is not the speed of particles of water The particular flux is a vector (greatness and course). Potential communicated as the stature of a section of water, has units of length .

About those vectors... Is the correct side of Darcy's law without a doubt a vector? h is a scalar, yet  H is a vector Since K is a tensor (yowser), K  H is a vector So all is well on the correct hand side Notes on K: we could likewise get a vector on the correct hand side by choosing K to be a scalar, which is frequently done (i.e., expecting that conductivity is autonomous of course).

A couple words about the K tensor K abdominal muscle relates angles in potential in the b-heading to flux that outcomes in the a-bearing. In anisotropic media, inclinations not adjusted to bedding give flux not parallel with potential slopes. In the event that the facilitate framework is adjusted to headings of anisotropy the "off corner to corner" terms will be zero (i.e., K abdominal muscle =0 where a  b). In the event that, likewise, these are all equivalent, then the tensor crumples to a scalar. The motivation to utilize the tensor frame is to catch the impacts of anisotropy. flux in x-course flux in y-bearing flux in z-heading

Looking comprehensively Check out the naturally parts of Darcy's outcome. The rate of stream is: Directly identified with the zone of stream (e.g., put two sections in parallel and you get double the stream); Inversely identified with the length of stream (e.g., move through double the length with a similar potential drop gives a large portion of the flux); Directly identified with the potential vitality drop over the framework (e.g., twofold the vitality exhausted to get double the stream). The expression is patently straight; all properties scale directly with changes in framework strengths and measurements.

Why is Darcy Linear? Since non-turbulent? No. Far before turbulence, there will be huge neighborhood increasing speeds: it is the absence of nearby quickening which makes the relationship straight. Consider the Navier Stokes Equation for liquid stream. The x-part of stream in a speed field with speeds u, v, and w in the x, y, and z (vertical) headings, might be composed

Creeping stream Now force the conditions required for which Darcy's Law "Crawling stream"; quickening (du/dx) terms little contrasted with the thick and gravitational terms Similarly changes in speed with time are little so N-S is: Linear in inclination of water driven potential on left, corresponding to speed and thickness on right (same as Darcy). Verification of Darcy's Law? No! Demonstrates that the crawling stream suspicion is adequate to acquire amend frame.

Capillary tube display for stream Widely utilized model for move through permeable media is a gathering of tube shaped fine tubes (e.g.,. Green and Ampt, 1911 and some more). We should infer the condition for unfaltering course through a hairlike of range r o Consider constrains on tube shaped control volume demonstrated  F = 0 [2.75]

Force Balance on Control Volume end weights: at S = 0 F 1 = P  r 2 at S =  S F 2 = (P +  S dP/dS)  r 2 shear force: F s = 2  r  S  where  is the neighborhood shear push Putting these in the drive adjust gives P  r 2 - (P +  S dP/dS)  r 2 - 2  r  S  = 0 [2.76] where we recall that dP/dS is negative in sign (weight drops along the heading of stream)

proceeding with the compel adjust With some polynomial math, this rearranges to dP/dS is consistent: shear stretch differs straightly with sweep From the meaning of thickness Using this [2.77] says Multiply both sides by dr, and coordinate P  r 2 - (P +  S dP/dS)  r 2 - 2  r  S  = 0 [2.76]

Computing the flux through the pipe... Completing the incorporation we discover which gives the speed profile in a barrel shaped pipe To ascertain the flux coordinate over the territory in round and hollow directions, dA = r d  dr, in this manner

Rearranging terms... The basic is anything but difficult to figure, giving ( fourth power!!) which is the notable Hagen-Poiseuille Equation. We are occupied with the stream per unit zone (flux), for which we utilize the image q = Q/ r 2 (second power) We regularly measure weight as far as water driven head, so we may substitute r gh = P, to acquire

r 0 2/8 is a geometric term: capacity of the media. alluded to as the inborn penetrability , signified by  .  is a component of the liquid alone NOTICE: Recovered Darcy's law! See why by hauling  out of the pressure driven conductivity we get an inborn property of the strong which can be connected to a scope of liquids. SO if K is the immersed water powered conductivity, K=   . Along these lines we can ascertain the powerful conductivity for any liquid. This is extremely valuable when managing oil slicks ... bubbling water spills ..... and so on .

Darcy's Law at Re# > 1 Often noticed that Darcy's Law separates at Re# > 1. Laminar stream holds vessels for Re < 2000; Hagen-Poiseuille law still substantial Why does Darcy's law separate so soon? Laminar closures for characteristic media at Re#>100 because of the tortuosity of the stream ways (see Bear, 1972, pg 178). Still far over the esteem required for the infringe upon down of Darcy's law. Genuine Reason : because of powers in speeding up of liquids passing particles at the infinitesimal level being as vast as thick strengths: expanded imperviousness to stream, so flux reacts less to connected weight slopes.

A couple of more words about Re#>1 Can figure out this through a straightforward count of the relative sizes of the thick and inertial strengths. F I  F v when R e #  10. Since F I run with v 2 , while F v runs with v, at Re#  1 FI  Fv/10, a sensible cut-off for crawling stream estimate

Deviations from Darcy's law (a) The impact of inertial terms getting to be distinctly noteworthy at Re>1. (b) At low stream there might be an edge inclination required to be overcome before any stream happens whatsoever because of hydrogen holding of water.

How does this apply to Vadose? Consider run of the mill water stream where v and d are expanded Gravity driven stream close immersion in a coarse media. most extreme neck distance across will be around 1 mm, vertical flux might be as high as 1 cm/min (14 meters/day). [2.100] Typically Darcy's OK for vadose zone. Could have issues around wells

What about Soil Vapor Extraction? Does Darcy's law apply? Air speeds can surpass 30 m/day (0.035 cm/sec). The Reynolds number for this wind current rate in the coarse soil utilized as a part of the case considered above is [2.101] once more, no issue, in spite of the fact that stream could be higher than the normal mass stream about bays and outlets

Summary of Darcy and Poiseuille For SATURATED MEDIA Flow is straight with penetrability and inclination in potential (main thrust) At high stream rates gets to be non-direct because of neighborhood speeding up Permeability is because of geometric properties of the media (inborn porousness) and liquid properties (consistency and particular thickness) Permeability drops with the square of pore size Assumed no slip strong fluid limit: doesn't work with gas.